Suicide hijackers may have been on board a fifth American transcontinental plane on the day of the attacks on the Pentagon and the World Trade Center, sources close to the FBI investigation have reportedly said.

One source told The Chicago Tribune that the FBI was searching for a number of passengers who were due to fly on American Airlines Flight 43 from Boston, which was grounded due to a mechanical problem.

There were other acts of terrorism in the United States and elsewhere that were part of this plan

Florida Senator Bob Graham

The plane had been scheduled to take off at 0810 local time, just 25 minutes after American Airlines Flight 11, which struck New York's World Trade Center.

One of the sources told the newspaper that the FBI was also "very interested" in people whose names appeared on the passenger lists of several other American flights which were in the air when the first attacks occurred.

Those planes were then prematurely landed under the orders of air traffic controllers in response to the attacks on the World Trade Center and the Pentagon.

Series of attacks

None of the passengers being sought by the FBI reappeared to board the same, rescheduled flights when the grounding order on commercial planes in the US was lifted last week.

Florida Senator Bob Graham, chairman of the Senate Intelligence Committee, said the hijackings were intended to be the first in a series of global terror attacks.

The FBI has warned the US could still be at risk

"There has been credible evidence gathered since Tuesday that the attacks were not designed to be a one-day event," Senator Graham told the Orlando Sentinel.

"There were other acts of terrorism in the United States and elsewhere that were part of this plan," he said. Those were not necessarily other hijackings, but could have been terror tactics such as "putting a chemical in a city's water system, or blowing up a bridge in a major urban centre," he explained.